Since the market has been demanding ever cheaper and better quality flat panel displays, including PDPs, it has become necessary to develop new processes for each step of the manufacturing chain to enable the production of less expensive and better quality products. One technology utilized in PDP manufacturing has been thick film technology, including photoimageable thick films and photo-patterning thick film technologies. Photo-patterning technologies offer a means of making uniform finer lines and spaces compared to traditional screen-printing methods.
A photo-patterning method, such as Du Pont's FODEL® (E.I. du Pont de Nemours and Company) printing system, utilizes a photoimageable organic medium, as found in U.S. Pat. Nos. 4,912,019; 4,925,771; and 5,049,480, herein incorporated by reference, whereby the substrate is first completely covered (printed, sprayed, coated or laminated) with the photoimageable thick film composition and dried, if necessary. An image of the pattern is generated by exposure of the photoimageable thick film composition with actinic radiation through a photomask bearing a pattern. The exposed substrate is then developed. The unexposed portion of the pattern is washed away leaving the photoimaged thick film composition on the substrate that is then fired to remove organic materials and sinter inorganic materials and to adhere the pattern to the substrate. Such a technique has been widely used in the manufacture of plasma display panels.
The organic basis of typical thick film pastes is termed a medium. A medium consists of one or more polymers dissolved in a solvent. Such polymers impart a number of properties to the paste, including viscosity and dried toughness and strength. Other materials can be added at the medium stage, including, but not limited to, rheological additives, photoinitiators, and other materials.
The commonly used and existing process of making photoimageable thick film pastes is to dissolve a pre-made polymer in a solvent, add photoinitators and other organic additives to form a photoimageable organic medium. Then, the organic medium is mixed with functional particles, such as conductive, resistive or dielectric particles, glass frits and other additives. The polymer used to make the organic medium is separately pre-made by polymerization of monomers in a low boiling point solvent, such as 2-propanol or methylethylketone, followed by removing the solvent and drying to form a polymer powder. Next, for thick film applications, the polymer powder is re-dissolved in a high boiling point solvent commonly used for thick film pastes, such as Texanol® or terpineol. The process is time-consuming, involves multiple steps, and tends to introduce many variables including potential contamination which may lead to poor quality of the final photoimageable thick film pastes.
The present invention provides a manufacturing process for thick film pastes, including photoimageable thick film pastes, by providing a novel method of forming what is termed a polymer solution. This novel method of forming a polymer solution may be directly used in a new process to manufacture thick film pastes. Essentially, the present invention provides a new method for the formation of a thick film paste, wherein manufacturing steps are reduced because there is no need to pre-make and handle the dried polymer and re-dissolve the polymer to form an organic medium.